In recent years, the issues of refrigerant substitution and refrigerating system efficiency improvement have been studied from the standpoint of minimizing ozone layer destruction and global warning. In the area of refrigerant substitutes, progress is being made in the substitution of HFCs (hydrofluorocarbons) for chlorine-containing refrigerants such as CFCs (chlorofluorocarbons) and HCFCs (hydrochlorofluorocarbons) On the other hand, since HFC refrigerants could be subject to restrictions in light of the problem of global warming, natural refrigerants such as carbon dioxide, ammonia and hydrocarbons are also being researched for applied use.
Efforts toward such refrigerant substitution are advancing in parallel with development of refrigerating machine oils for these substitute refrigerants. Refrigerating machine oils must satisfy a number of performance requirements including lubricity, miscibility with refrigerants, heat and hydrolytic stability, electric insulating property and low hygroscopicity, and therefore compounds satisfying these requirements are selected to match the type and purpose of use of each refrigerant. Examples of refrigerating machine oils used for HFCs include oxygen-containing compounds such as esters, ethers and carbonates that are miscible with the refrigerants, and alkylbenzenes which have inferior miscibility with the refrigerants but have excellent lubricity and heat and hydrolytic stability.
At the same time, efforts are being made to lower the viscosity of refrigerating machine oils with the goal of achieving higher efficiency of refrigerating systems. Known ester-based refrigerator oils include polyol esters obtained by reaction of aliphatic polyhydric alcohols and fatty acids, as disclosed in Japanese Translation Publication No. HEI 3-505602 (JP-A 3-505602) of International Publication for Patent Application and Japanese Patent Kokai (Laid-Open) Publication No. HEI 3-128991 (JP-A 3-128991), and for reduction of the viscosity of such ester-based refrigerating machine oils it has been found effective to select fatty acids with low carbon number alkyl groups for use in the raw material. However, fatty acids with lower alkyl groups generally produce the undesirable situation of low heat and hydrolytic stability of the obtained esters. On the other hand, fatty acids with high carbon number alkyl groups are selected in order to increase the viscosity of such esters, but this creates a problem in that sufficient miscibility with refrigerants cannot be achieved.
There are also known alicyclic polycarboxylic acid esters, such as disclosed in Japanese Patent Kokai (Laid-Open) Publication No. HEI 9-221690 (JP-A 9-221690), as ester-based refrigerating machine oils with excellent heat and hydrolytic stability, but those with a large number of carbon atoms in the terminal alkyl group at the ester site have insufficient miscibility with refrigerants, while those with a small number of carbon atoms in the terminal alkyl group have inferior heat and hydrolytic stability, as well as insufficient lubricity.
There has yet to be developed, therefore, an ester-based refrigerating machine oil that has a satisfactory balance of lubricity, heat and hydrolytic stability and refrigerant miscibility, while also satisfying the other required aspects of performance such as electric insulating property.